The present disclosure relates to light guides and illumination devices and particularly relates to a technique for reflecting light, which has entered a light guide, toward the exit surface of the light guide.
Some image forming apparatuses, such as multifunction peripherals, employ as a light source section for an image reading device, such as a scanner, a line light source includes a resin-made light guide in which a rod-shaped, and an LED capable of emitting light to the interior of the light guide through a longitudinal end surface of the light guide. The light source section is required to linearly illuminate an original document to be read in synchronism with reading of a line sensor. For this purpose, a surface of the light guide opposite to an exit surface thereof through which light exits the light guide is provided with light reflecting or scattering patterns, so that the patterns can deflect light, which has entered the light guide, toward the exit surface to allow linear illuminating light to exit the light guide in the direction toward the original document.
Such light source sections include those of double-sided illumination type which are configured to illuminate an original document to be read from both sides in a sub-scanning direction (a direction perpendicular to a main scanning direction of the light source section). The light source sections further include those of one-sided illumination type configured to illuminate an original document to be read only from either side in the sub-scanning direction. The light source sections of double-sided illumination type offer superior reading performance because of illumination onto an object of reading from both sides. The light source sections of one-sided illumination type are more favorable in cost terms than those of double-sided illumination type, although if the object of reading is an uneven original document (an original document having a piece of paper attached thereto or an original document having a non-flat shape), they are likely to provide shadows corresponding to recesses in the document having not been illuminated
An example of the light reflecting or scattering pattern is a prism provided in the interior of a cylindrical light guide made of a light transmissive material. The prism is provided on a reflective surface of the light guide opposite to an exit surface (texture surface) thereof and allows incident light from a light source (such as a white LED), which has entered the light guide through an incident surface thereof located at one longitudinal end, to be reflected toward the exit surface. An example of the prism used is an oval hemispherical prism having excellent diffusion of exit light in the sub-scanning direction.